This non-provisional application claims priority under 35 U.S.C. § 119(a) on Korean Patent Application No. 2003-63229 filed on Sep. 9, 2003, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to an improvement in the luminescent efficiency in semiconductor nanocrystals by surface treatment, and more particularly to a method for improving the luminescent efficiency of semiconductor nanocrystals, prepared by wet chemistry method, without changing the luminescent characteristics of the nanocrystals such as luminescence wavelengths and the distribution thereof.
2. Description of the Related Art
When semiconductor compound material is manufactured into nano-sized crystals (nanocrystals), quantum confinement effects are exhibited in the range shorter than the bulk exciton Bohr radius of the compound semiconductor material. Due to the quantum confinement effects, the characteristic energies corresponding to the respective band gaps of the semiconductor materials are changed. When a semiconductor compound material capable of emitting visible light is manufactured into nanocrystals, the band gap energies of the semiconductor nanocrystal compounds begin to increase and consequently blue-shift, whereby it is observed that the luminescent region is shifted toward the blue region as the nanocrystal size decreases below a particular size. Since the control over the characteristics, structure, shape and size of the semiconductor nanocrystals enable a control of the corresponding band gaps, energy levels over a very broad range can be obtained.
In recent years, there have been many attempts to grow nanocrystals in varied sizes by a wet chemistry method wherein a precursor material is deposited with a surfactant in a hot coordinating organic solvent. According to the wet chemistry method, as the nanocrystals are grown, the organic solvent is naturally coordinated to the surface of the nanocrystals and acts as a dispersant. Accordingly, the organic solvent allows the initial nucleus to grow to the level of nano-sizes. In addition, the wet chemistry method has the advantage in that the size of nanocrystals can be controlled by changing the concentration of the precursors, the kind of organic solvents, the synthesis temperature and time, and the like. However, the size of the nanocrystals to be synthesized is very small, and the surface area relative to the volume of the nanocrystal is increased, causing defects on the surface. Since these defects act as energy traps between energy band gaps, they degrade the luminescent efficiency of the nanocrystal. Moreover, the smaller the nanocrystals, the more serious the problem.
Methods reported heretofore for improving the luminescent efficiency of nanocrystals are largely divided into the following two processes.
The first process is a surface passivation process wherein a stable organic or inorganic material is coated on the surface of nanocrystals to form a protective film thereon. The luminescent efficiency of the nanocrystals varies depending on the kind of organic dispersant surrounding the surface of the nanocrystals. In this connection, it was reported that when allylamine or dodecylamine had been substituted for trioctyl phosphonic acid on the surface of CdSe nanocrystals, the nanocrystals exhibit 40˜50% -improvement in luminescent efficiency (Nano letters, 2001, 1, 207-211). Based on the fact that inorganic protective films exhibit excellent stability and distinct effects compared to organic protective films, a great deal of research on the inorganic protective films has been conducted. Nanocrystals generally have a structure consisting of a core portion as substantial nanocrystals and a shell portion as an inorganic protective film. Core-shell structured nanocrystals exhibiting improved luminescent efficiency and the method for preparing the nanocrystals are disclosed in U.S. Pat. Nos. 6,322,901 and 6,207,229. The core-shell structured nanocrystal was reported to exhibit improved luminescent efficiency by 30˜50%. However, since the preparation method involves a troublesome coating step and since the luminescence wavelengths and size distribution of the nanocrystals may be varied during coating, it has the drawback of broadening the luminescence wavelength distribution. In addition, due to the lattice mismatch between the core and shell portions and interface strain caused by the thickening shell, the luminescent efficiency of the nanocrystals may be degraded. Furthermore, the coating step is difficult to carry out and the reproducibility is poor. Moreover, the characteristics of materials constituting the core and shell portions limit the selection of the materials.
The second process is a synthesis of novel nanocrystals. The present inventor has developed nanocrystals with improved luminescent efficiency, presumably in alloy form, which can be synthesized in a simple way, and filed a patent application (Korean Patent Appln. No. 2003-0049547). The nanocrystals are prepared by mixing at least two precursors belonging to the same group and a precursor belonging to a different group, and adding the mixture to an organic solvent. The reaction between the precursors leads to the synthesis of three-component nanocrystals. The nanocrystals thus synthesized exhibit an improvement in luminescent efficiency and are prepared in a simple and easy way in comparison with the nanocrystals having a core-shell structure as discussed above. Similarly, there is a report that three-component nanocrystals can be prepared in the form of a homogeneous alloy or can have a gradient composition, depending on the mixing ratio of the precursors. In addition, there is another report wherein nanocrystals in alloy form and having improved luminescent efficiency were synthesized by annealing core-shell structured nanocrystals at high temperature (J. Am. Chem. Soc., 2003, 125, 8589-8594).
Although the nanocrystals discussed above exhibit an improvement in luminescent efficiency, there have been few reports regarding the luminescent efficiency of nanocrystals which can emit light, in particular, in the blue region (having higher energies). This fact suggests that the problems caused by energy traps formed on the surface of small size crystals still remain unsolved.